Remove storage limit column from application settings

What does this MR do?

This is a follow-up from !38108 (merged) and removes the application_settings.namespace_storage_size_limit column from the database. This MR is the second step to delete it.

Migrate

bin/rails db:migrate
== 20201102114018 RemoveStorageSizeLimitFromApplicationSettings: migrating ====
-- remove_column(:application_settings, :namespace_storage_size_limit)
   -> 0.0024s
== 20201102114018 RemoveStorageSizeLimitFromApplicationSettings: migrated (0.0024s)

Rollback

bin/rails db:rollback
== 20201102114018 RemoveStorageSizeLimitFromApplicationSettings: reverting ====
-- add_column(:application_settings, :namespace_storage_size_limit, :bigint, {:default=>0})
   -> 0.0038s
== 20201102114018 RemoveStorageSizeLimitFromApplicationSettings: reverted (0.0039s)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team